Prior art procedures for certain prepreg tape materials, specifically Torayca® 3631, 350 F curing toughened carbon/epoxy prepreg, have vacuum bag process limitations that cause excessively long processing times to complete reworks with patch thicknesses greater than 12 plies when the patch is at an edge and 6 plies when the patch is not at an edge. When the required rework exceeds these ply limitations, a partial patch (of either 6 plies or 12 plies depending on the circumstance) is applied and cured, including an adhesive layer. This process is repeated until the entire patch is built up to the required number of layers. As an example, a rework patch in the center of a part requiring 54 plies must be built up 6 plies at a time in 9 separate curing operations. The primary reason for this limitation is that excessive porosity results in the patch material when plies greater than the limitations are processed at one time. The excessive porosity is undesirable. The lay-up process includes 1-minute ply debulking for every ply to minimize trapped air between plies. In addition, the cure cycle includes an intermediate 60 minute hold at 200 F to minimize porosity in the patch. Both of these provisions add significant time to the overall process.
A standard vacuum bag and curing processes used in the prior art results in a patch or laminate that may have excessive porosity levels. Typical acceptable porosity levels are less than ≦4% for reworked parts with ≦2% for original manufacturing, whereas the Toray carbon/epoxy materials exhibit porosity levels in the range of 5-10% when processed outside of the production process limitations.
Additionally, having multiple layers of adhesive imbedded into the patch may be undesirable due to the resulting porosity in the adhesive layers. This result may be problematic for rework on thick laminates as the required strengths may not be able to be obtained using the current vacuum bag processes. Unless the part can be processed in an autoclave, a thick patch of toughened carbon/epoxy composites may not be possible with prior art techniques.
Development of a patch creation and debulking tooling approach is disclosed in U.S. Pat. No. 6,761,783 entitled PROCESS METHOD TO REPAIR BISMALEIMIDE (BMI) COMPOSITE STRUCTURES having a common assignee with the present application.
It is therefore desirable to provide a protocol for establishing process times and parameters for application to a standard rework approach. It is further desirable to employ the developed protocol for specific process parameter definition for patch creation in toughened carbon/epoxy prepreg materials.